My understanding is that the voucher must be used from your account, i.e. the Avios come
out of it. I have only been ever to use the voucher in conjunction with my EC account. For
example, I want to redeem to LAX in F for 150,000 Avios, I need to have 150,000 Avios in
order to use it. If I don't, then I need to either purchase some more, take a short trip or I
believe someone Avios rich could transfer some time me (for a fee).

As to the second question, travel must be booked before the expiration of the voucher. You
can fly out before expiration but come back long after. For example, it expires on 12.12.12
and you fly on 11.12.12 but return on 24.12.12. That's perfectly fine.

Those more intimately knowledgeable than I will come along and make any necessary corrections
to the above

H